Semiconductor components, such as packages, dice and wafers include external contacts which provide input/output paths to the integrated circuits contained on the components. For surface mount components, the external contacts typically comprise solder balls, or solder bumps, bonded to contact pads on the component. For some components, such as chip scale packages and BGA packages, the external contacts can be arranged in a dense grid array, such as a ball grid array (BGA), or a fine ball grid array (FBGA).
One conventional method for forming external contacts uses pre-formed solder balls, and a solder reflow bonding process. A prior art solder reflow bonding process is illustrated in FIGS. 1A-1D.
As shown in FIG. 1A, a semiconductor component 10 includes a pattern of contact pads 12 in electrical communication with the integrated circuits and semiconductor devices contained on the component 10. The contact pads 12 typically comprise a solder wettable metal such as chrome, titanium or nickel. The component 10 also includes a pattern of conductive traces 14 in electrical communication with the contact pads 12. In addition, the component 10 includes an electrically insulating solder mask 16 having openings 22 aligned with the contact pads 12. The solder mask 16 substantially covers the conductive traces 14, but as illustrated in FIG. 1A, the openings 22 sometimes expose portions of the conductive traces 14 that are in close proximity to the contact pads 12.
As shown in FIG. 1B, during the bonding process, a layer of flux 18 is deposited on the contact pads 12 using a suitable deposition process such as screen printing or pin transfer. The flux 18 chemically attacks surface oxides, such that the molten solder can wet the surfaces to be bonded. Typically, the flux has either a rosin based or a water soluble chemistry.
As shown in FIG. 1C, following application of the flux 18, pre-formed solder balls 20 can be placed on the contact pads 12 in physical contact with the flux 18. Typically, the solder balls 20 have the shape of a sphere, or a truncated sphere. Although a fixture can be used to center and maintain the solder balls 20 on the contact pads 12, the flux 18 also performs a tacking function for holding the solder balls 20 on the contact pads 12 during the bonding process.
Following placement of the solder balls 20 on the contact pads 12, the component 10 can be placed in a furnace at a temperature sufficient to reflow and metallurgically bond the solder balls 20 to the contact pads 12. The component 10 can then be removed from the furnace and cooled. The component 10 can then be surface mounted to a supporting substrate, such as a printed circuit board, by bonding the solder balls 20 to corresponding electrodes on the supporting substrate.
In addition to reflowing the solder balls 20, the high temperatures encountered in the furnace can cause most of the flux 18 to vaporize in the furnace. However, a flux cleaning step is also required to remove any flux residue that remains on the contact pads 12 and component 10. The flux cleaning process can be performed using a hydrocarbon for a rosin based flux, or water with surfactants for a water soluble flux.
One factor that can adversely affect the reliability of the component 10 is shorting caused by distortion of the solder balls 20. As shown in FIG. 1D, distortion of the solder balls 20 can cause portions of the solder balls 20 to touch exposed portions of the conductive traces 14. It would be advantageous for a flux to be curable and electrically insulating, such that shorting between the solder balls 20 and the conductive traces 14 can be eliminated.
Another factor that can affect the reliability of the component during normal operation is fatigue failure of the solder balls 20, particularly at the interfaces with the contact pads 12 and the electrodes on the supporting substrate. Typically, fatigue failures are induced by thermal expansion mismatches between the component and the supporting substrate. For example, if the component 10 comprises a first material, such as silicon having a first TCE (thermal coefficient of expansion), and the supporting substrate comprises a second material, such as FR-4 having a second TCE, cyclic forces can be placed on the solder balls 20 as the component 10 is thermally cycled during normal operation.
These forces can include tensile forces, moment forces and shear forces. If the forces are large enough, the solder balls 20 can separate from the contact pads 12 forming an electrical open. The separation can also compromise the physical bond between the component 10 and the supporting substrate. This problem is compounded because the area of interface between the solder balls 20 and the contact pads 12 is relatively small, such that the forces are concentrated over a relatively small area.
It would be advantageous for a flux to have the capability to support and rigidify the solder balls 20, or other external contacts, on a semiconductor component 10, during normal operation of the component 10. The present invention is directed to an improved flux configured to form a rigidifying structure for the external contacts. In addition, the flux is formulated to remove oxides from the components, is curable for reducing vapors during the fabrication process, and is electrically insulating for masking conductive traces for the external contacts.